High image rendering capabilities have been demanded for a display device represented by a television set and projector device. Especially, optimized design of a display device has been studied to implement technological means that provides a viewer with the feelings of presence and immersion. To implement these feelings of presence and immersion, standardizations have been progressed through optimization of the relationship between viewing angle characteristics of a man and display screen sizes. That is, attempts have been made to improve these image rendering capabilities by rendering images with a screen size that is suitable for human viewing angle characteristics.
Generally, visual field of a man is classified into some classes by visibility level. The classes are listed below in the order of viewing angle from the smallest.
(1) Discriminable visual field: High accuracy information perception range such as reading characters (within about 5 degrees in vertical and horizontal directions)
(2) Functional visual field: The range where information can be searched for by eye motions only (within about 10 degrees in vertical direction and about 15 degrees horizontal direction)
(3) Inducible visual field: The range where existence of information can barely be known, and the information therein influences one's sense of orientation (within about 40 degrees in vertical direction and about 50 degrees in horizontal direction)
The sense of image perception of a man will change when an image is rendered in each of visual field ranges. That is, the further the image rendering range extends from the functional visual field, the more feeling of presence one can get. Furthermore, if the image rendering range extends from the inducible visual field, the feeling of immersion can be obtained. Thus, the larger a perceived image occupying area becomes within the visual field of a man, the (virtual) feeling of reality impressed from the image will increase gradually.
Due to these reasons, large screen implementation technologies are useful to induce visual sense characteristics of a man. On the other hand, for rendering images with further reality, there still remains an issue on how to provide stereoscopic feeling with the images rendered on a planar screen. For example, with a so-called hold-on display type display device which is represented by a liquid crystal display providing a liquid crystal panel with light having uniform brightness on a screen, three-dimensional image rendering capability is not sufficient. This is because, for example, when two objects are to be displayed on a screen, they are rendered with the same brightness, irrespective of their distance or depth perception.
For example, in rendering an image of one white ball, the stereoscopic effects of a ball differ significantly when the ball image area is rendered with the same whiteness and brightness and when the ball image area is rendered with gradient applied uniformly from a certain point within the area. That is, in order to output an image with stereoscopic effect and perspective, a method to provide suitable distribution in brightness of the object to be recreated may be a solution, however, complexity of image signal processing has prevented the technology from being progressed.
As described above, representation of stereoscopic effect and feeling of presence is not sufficient with a conventional hold-on display type display device compared with a display device using a cathode-ray tube, as the brightness distribution is uniform over the whole screen. The range of an image a man gazes at is within the aforementioned functional visual field at best where information perception is enabled by eye motions, thus the central portion of the screen and in the vicinity thereof. If the brightness of the peripheral portion of the screen excluding the gazing point is equals to or higher than the brightness of the central portion which is the gazing point, a man is bothered by visually uncomfortable feeling, and becomes tired easily. That is, like a display device using cathode-ray tube, rendering of an image that does not cause uncomfortable feeling nor make a man tired will be enabled by increasing the brightness at the central portion and in the vicinity thereof relatively, compared with the brightness at the peripheral portion of a screen.
FIG. 27 shows an example of relative brightness distribution characteristics in horizontal direction of a screen in a display device having a cathode-ray tube. The length in horizontal direction is divided equally to 20, and the relative brightness distribution in horizontal direction is shown with the brightness at the center (the position with a scale of 11 in the figure) being scaled to 100. Generally, the brightness provided by a cathode-ray tube at both sides is in a range of 65 (relative brightness distribution characteristic 101 in the figure) to 85 (relative brightness distribution characteristic 102 in the figure), which is relatively low, compared with the brightness of 100 at the center. This is because the deflection center of an electron beam through a horizontal and vertical deflection circuit of a cathode-ray tube is located before the center of the curvature of a screen, and the distance from the deflection center of the cathode-ray tube is longer at the peripheral portion of the screen. According to the brightness distribution characteristics of a display device using this cathode-ray tube, however, the brightness at the central portion of the screen and in the vicinity thereof is relatively higher than the brightness of the peripheral portion of the screen, enabling to render images within the functional visual field, that are not uncomfortable for a man and that do not make a man easily get tired.
In the hold-on display type liquid crystal display device described above, a backlight unit is used as the unit to illuminate an object such as a liquid crystal display panel. In a liquid crystal display device, two types of structures have been adapted: direct type and edge light type (light guide plate type) backlight units.
The direct type backlight unit is a system arranging fluorescent tubes and/or LEDs (Light Emitting Diodes) that are the light sources disposed directly under the liquid crystal panel that is an object, and sufficient brightness can be obtained by increasing the number of light sources in accordance with the screen size of the display device. However, uneven brightness is easily caused between the portions where light sources are disposed and the other portions where light sources are not disposed. With a direct type backlight unit, strength of the backlight unit is required to be attained. Thus, for example, a structure having a backlight unit enclosure formed by a metal plate, a reflective sheet applied on the inner surface of the backlight, and a plurality of straight tube lamps arranged thereon has been employed.
The edge light type backlight unit, on the other hand, is a system where light sources such as fluorescent lamps are arranged at the edge portion of a light guiding body made from transparent acrylic plate or another, so that one single panel is formed as the surface light source by utilizing multiple reflection within the light guiding body. With an edge light type backlight unit, a reflector is disposed behind strip lamps or L-shaped lamps. The display device using the edge light type backlight unit can reduce the thickness, however, the mass of the light guiding body becomes excessively large with a large model, and the brightness on the screen becomes difficult to be attained as the screen gets larger.
Due to the characteristics described above, generally direct type backlight unit is used for a large screen liquid crystal display device, while an edge light type backlight unit is used for a small screen liquid crystal display device.
Among liquid crystal display device with backlight unit described above, a liquid crystal display device is disclosed, wherein the clearance between fluorescent tubes is smaller at the central portion of the screen, and is getting larger toward the edges (for example, see Patent Document 1) in order to achieve lower power consumption, compactness, thinness and lightness. The device is so designed as to decrease the number of backlight fluorescent tubes by reducing the brightness gradually from the center of a screen toward the edges, while brightness is maintained at a level a man cannot recognize the reduction of the brightness.
A backlight illuminating a liquid crystal panel is disclosed, wherein the brightness is maximum at a position above or below the center and is gradually reduced in the vertical direction (for example, see Patent Document 2). This is achieved by controlling the brightness distribution of the backlight through adjustment of a reflector or control of the open width of the color filter.
Yet another backlight unit is disclosed, wherein the uniformity of the surface light source brightness is maintained by setting the clearance between straight tube type lamps at the central portion of the display screen of a liquid crystal panel smaller, and wider toward the edges of the display screen, thereby achieving lower power consumption (for example, see Patent Document 3).
The backlight units disclosed in the above-mentioned Patent Documents 1 and 3 are designed to reduce power consumption by reducing the number of fluorescent lamps constituting a backlight while maintaining the uniformity of brightness on the display screen. The backlight unit disclosed in Patent Document 2 implements uniformity of display by correcting non-uniformity of brightness in vertical direction that is caused by angle-dependent visibility and heat created by the backlight by adjusting the brightness distribution in vertical direction of the screen of a liquid crystal panel.
Thus, the backlight units disclosed in above mentioned Patent Documents 1, 2 and 3 consequently provides brightness distribution on the display surface of a liquid crystal display device, however, this brightness distribution is formed only in the vertical direction of the display screen. However, the backlight units do not implement two-dimensional characteristics in accordance with visual sense characteristics of a man as provided by a display device using a cathode-ray tube, that is, brightness distribution characteristics with gradient being formed at least in the horizontal and vertical directions from the central portion of a screen. Therefore issues of visual uncomfortableness and fatigability for a man have not been improved yet.
The present invention is conceived considering the aforementioned situation, and is intended for providing, in accordance with visual sense characteristics of a man, a backlight unit that enables rendering of images that are not uncomfortable to a man and not easily fatigable by forming brightness gradient at least in the horizontal and vertical directions so that the brightness of the central portion of the screen and in the vicinity thereof is relatively higher than the brightness at the peripheral portion, which is provided by a display device with a cathode-ray tube, and also a liquid crystal display device using the backlight.
Patent Document 1: Japanese Laid-Open Patent Publication No. H06-75216
Patent Document 2: Japanese Laid-Open Patent Publication No. H11-119217
Patent Document 3: Japanese Laid-Open Patent Publication No. 2002-82626